The present invention relates to an improved circuit arrangement for the digital indicator of the received frequency of a radio receiver operating in both the AM and FM frequency ranges.
U.S. Pat. No. 3,949,307, issued Apr. 6, 1976, the subject matter of which is incorporated herein by reference discloses a circuit arrangement for digital frequency indication in an AM/FM radio receiver in which circuit arrangement the oscillations of the AM and FM tuning oscillators are amplified and converted to rectangular oscillations or signals whose frequency is counted by means of electronic counters and then digitally displayed. Since the respective signals obtained from the FM oscillator or from the AM oscillator are too weak to actuate the counting circuits, it is necessary to amplify them. The amplified oscillations or output signals of the respective oscillators are additionally converted to rectangular signals of the same fundamental frequency which are suitable to actuate the digital counting circuits.
In the above-identified United States patent, the oscillations of the AM oscillator and those of the FM oscillator are amplified by means of a common broadband amplifier which has a sufficiently broad bandwidth that the oscillator oscillations for the AM range as well as those for the FM range are covered. The couplings of the broadband amplifier to the oscillators are dimensioned so loosely that the oscillators will not noticeably be detuned by changes in capacitance of the broadband amplifier.
It has been found, however, that the expenditures for the common broadband amplifier to amplify and rectangularize the oscillations are relatively high.